Having owned previous generations of BOTH NAD "74x" & "75x" series, Marantz "500x" & "600x"...I give the nod to Marantz.
You will be pleased with sound quality of both receivers. However, NAD 747 is more comparable to Marantz 5005. Marantz 6005 compares to NAD 755.
Marantz arguably has better reliability more relevant HT features dollar for dollar.
For price of Marantz 5005, I can add an external 2 channel amp performance that will BLOW away both "747" & "6005"...something to think about.